What is the difference with the progress meter between a PW and a Kindle 4 NT?

I don't know. I come straight from the K3 (KK).

There we have a bar along the bottom with 'blobs' representing the chapters, that gets filled in as you read the book. I found it a nice way of seeing roughly how much is left in the current chapter, as well as how many chapters are remaining, all in one neat graph.